The optical design of a progressive addition eyeglass lens (PAL) involves steadily changing the power (inverse focal length) between different regions of the lens, giving the lens an image quality that changes over the lens surface. This change introduces large amounts of astigmatism along with significant amounts of other aberrations. Thus, a PAL, even if manufactured to reproduce the optical design perfectly, will have significant variations in image quality across the lens. Moreover, eyeglasses in general have defects associated with their manufacture, such as deviation from design curvatures, refractive index variations, bubbles, inclusions, and scratches which may also alter image quality. While these manufacturing defects are difficult to model, it is necessary to be able to measure them. With the rapidly-growing popularity of PAL's, a need exists for an optical testing apparatus that measures the variation in image quality of different designs to provide an objective comparison.
Currently, measurements of power, astigmatism and prism can be made manually at certain locations on a lens (generally with about a 3 millimeter subaperture diameter) using commercially-available lensometers, otherwise known as focimeters. Measurement of resolution is typically made using a test bench setup through the inspection of an image formed by a
of an Air Force bar target object at infinity using a microscope. While these instruments and methods do provide some basic information about the PAL, they fail to give precise, comprehensive image quality information that would be very useful to the PAI designer and/or manufacturer. Moreover, known instruments and methods do not allow for fully automated mapping of the power, astigmatism, prism and Modulation Transfer Function (MTF) over the entire surface of a PAL.
Consequently, a need exists for an improved optical testing instrument, and more particularly, an optical testing instrument that allows for fully automated mapping of the power, astigmatism, prism and MTF over the entire surface of a PAL.